1. Field of the Invention
The present invention relates in general to a vibration reduction mechanism, and in particular to a torsional damper for use in the propulsion system for a watercraft.
2. Description of Related Art
In many propulsion systems that propel small watercraft, such as personal watercraft and small jet boats, an internal combustion engine drives a jet propulsion unit. For this purpose, a flexible coupling often couples an output shaft of the engine to the impeller shaft of a jet pump unit. The impeller shaft in turn drives a propulsion device, such as an impeller within the jet pump unit.
The propulsion systems of most small watercraft today use two or three cylinder engines. Such engines, however, can be prone to internal vibrational problems. For instance, the explosive forces acting on the pistons and the inertia forces of the moving parts vary in intensity as the pistons of the engine reciprocate. The resulting variation of force or torque on the crankshaft causes the crankshaft to twist in an oscillatory fashion so as to transmit torsional vibration. Torsional vibration is more noticeable at certain speeds and can be irritating to the rider. Such vibrations may also damage the crankshaft. That is, as the shaft winds up and relaxes under the oscillatory torsional force, alternating stresses internal to the shaft occur. Such stresses can cause failure under some conditions.
Prior watercraft propulsion systems have not adequately isolated the watercraft hull from such vibration, nor have they included a suitable mechanism to shift the severity of the resulting resonance. Vibration-absorbing engine mounts have been used in an attempt to lessen vibration transmission from the engine to the watercraft hull. Although somewhat effective in reducing the engine vibration felt by the rider, such mounts do not dampen torsional vibrations within the crankshaft.